This invention relates to pneumatic pressure probes for use, in particular, though not exclusively, in measuring change in intra-ocular pressure and in measuring pulsatile ocular blood flow.
More specifically this invention is concerned with pneumatic pressure probes, also known as pneumotonometers. These are similar to applanation tonometers which operate on the principle of applanation, or flattening, of the cornea in order to monitor pressure changes in the eye. (In fact pneumotonometers tend to indent the cornea as well as flatten it, but the magnitude of this indentation is microscopic). As the pressure in the eye changes the pressure required to applanate the cornea changes proportionately. When used in combination with a pressure sensing device, which may, for example, be incorporated in a computerised tonograph, analogue electronic signals representing continuous intra-ocular pressure changes may be produced and processed in order to calculate a value representing the pulsatile blood flow through the eye.
It has been demonstrated that ocular blood flow is of significant importance in research into, and also potentially for diagnosing and monitoring treatment of, many ocular conditions including glaucoma, diabetic retinopathy, macular degeneration and arteriosclerosis,as well as vascular abnormalities such as carotid stenosis.
European Patent number EP 0315329 describes an applanation-type pneumatic pressure probe for use in measuring fluid pressure within an organ such as an eye. This probe comprises a hollow handle defining a pressure chamber, a hollow shaft which is slidably received within the handle, and a sensor head including a contact face, mounted on a distal end of the shaft. A proximal end of the shaft is received in the pressure chamber but is not influenced by the pressure within the chamber. However, this probe design has many inherent performance problems associated with, in particular, undesirably high friction levels between the hollow handle and the piston. The piston is relatively long in comparison to the sensor head, which is itself relatively large and heavy in relation to the piston, especially where the piston and sensor head are made largely of metal. Furthermore, many problems are associated with the relatively high weights of the component parts of the probe and the relative dimensions of these parts.
Further problems with this known probe design arise from the fact that the piston is prone to damage, especially by bending, due to its relatively long length. A bent probe will not move freely in relation to the shaft, resulting in very poor measurements.
In order to obtain sufficiently accurate measurements, the contact face of the sensor head should be aligned substantially parallel to the applanated cornea, substantially perpendicular to the visual axis of the patient. However, with no means to ensure that the patient is correctly aligned with the probe and sensor head, variable results are often obtained due to poor alignment.
Furthermore, this known probe must be used in combination with a removable membrane mounted on the sensor head, in order to avoid direct contact between the patient's eye and the contact face of the sensor head (for hygiene reasons). Every time the membrane must be removed for cleaning, subsequent replacement of the membrane often results in incorrect seating of the membrane on the sensor head. Since the seating of the membrane is critical to the measurements obtained from the probe, with no means for guaranteeing that the seating of the membrane is constant, the repeatability of probe measurements cannot be ensured.